Thermoplastic materials including polyethylene, polyvinyl chloride, nylons, polypropylene, acetals, and the like have become increasingly important as materials of construction because of their moldability, ease of handling, light weight, and ability to withstand corrosion.
In many applications however, mechanical joints such as threaded joints are unacceptable because they are not fluid-tight. This is particularly of concern in the handling of hazardous materials and waste.
Numerous techniques have therefore been developed for the joining of thermoplastic materials including welding and the use of adhesives. The use of adhesives or solvents for making joints in plastic pipe, etc. is inconvenient in field applications where conditions are not ideal and where the time and other factors required to develop the maximum bond strength may not be available. Moreover, it may be impossible to find a suitable adhesive for the thermoplastic material.
Polyethylene has in the past been welded by techniques such as extrusion welding using a tool which applies heat to overlapped sheets of the thermoplastic material, and extrudes a bead of thermoplastic material at the edge of the lap. The bead of thermoplastic material is intended to fuse with the preheated sheets of the thermoplastic to form a liquid fusion of the two sheets and the extrudate to weld the overlapped sheets. This technique has not proven entirely satisfactory in practice because of a lack of uniformity in the welding process especially where hand held extrusion welding tools are used on large sheets of material.
Another technique known in the prior art involves the use of a so-called "speed-tip" for welding sheets of thermoplastic material in which a welding rod of the thermoplastic material is fed through the nozzle of a hot air gun. The stream of hot air together with the melted thermoplastic are applied to the area to be welded. Once again problems of uniformity and completeness of weld are evident. Frequently such welds include pinholes or unwelded portions and can also include portions in which excess heat has been applied and the thermoplastic material damaged. Obviously, damaged and incomplete welds are most undesirable.
A further technique utilizes a hot wedge which is passed bewteen the sheets of thermoplastic material to heat the opposite faces which are then pressed together to form a bond.
A number of electrically heated techniques for forming either lap or butt welds between two members of a thermoplastic material have been attempted in the past.
One technique involves the use of a bare wire or metal strip which is placed between the two members of thermoplastic material. This method is disclosed in the teachings of the following U.S. Pat. Nos.: 2,243,506 (Mitchell), 2,647,072 (Smith), 2,742,390 (Beck), 2,974,566 (Hurley), 3,049,465 (Wilkins), 3,061,503 (Gould), 3,348,640 (Thompson), 4,416,713 (Brooks), 4,176,274 (Lippera), and 4,375,591 (Sturm).
The present inventors have had experience in the welding of thermoplastic members without the supply of new material and have found that the wire can actually burn holes in the thermoplastic material of the members being welded. Accordingly it is very important to supply new material when welding two thermoplastic members.
Some other forms of welding rods have been disclosed in which wire or metal strips are coated in a relatively thick layer of thermoplastic material. This type of welding rod is disclosed in the following patents: U.S. Pat. No. 2,983,306 (Resnick), U.S. Pat. No. 3,506,519 and CA 837,562 (Blumenkranz), U.S. Pat. No. 3,378,672 and CA811,837 (Blumenkranz), and FR 1.072.800.
The present inventors have found that this type of welding rod results in the element being too remote from the surfaces to be welded. The plastic immediately surrounding the wire or metal strips burns before the material of the thermoplastic members melts. The weld, if formed, is of poor quality.
French patent number 1.072.800 also describes the use of an adhesive to hold the welding rod in place on one sheet before applying the second sheet and welding the sheets together. The present inventors have tried this method of holding the welding rod in place. They have found that the use of adhesives or double-sided tape adversely affects the integrity of the weld because the adhesive and double-sided tape act as a contaminants. These contaminants prohibit the intermolecular bonding that is desired between the thermoplastic members being joined and the welding rod.
Many of the prior art techniques have only limited applications and are limited to the welding of certain shapes.